1. Field of the Invention
The present invention relates generally to manufacture of heads for data storage devices and more specifically to a write head for a hard disk drive.
2. Description of the Prior Art
As the areal data storage density of magnetic media steadily increases and track widths become narrower and closer together, there is more and more chance of interference from adjacent tracks. This interference has become so common that the acronym “ATI” for Adjacent Track Interference has been coined. This interference naturally increases write and read errors, and is therefore undesirable. The design of write heads in general is a balance between producing poles which are narrow enough to prevent ATI and having poles broad enough to allow adequate magnetic flux flow to write or overwrite data satisfactorily. As track widths become narrower, this balance becomes ever more delicate. In an effort to produce more narrow pole tips, channel shrinking films, as discussed below, have become more widely used in the fabrication process.
A typical read/write head 14 is shown in FIG. 4, which is a side cross-section view of the slider 16 shown in FIG. 3. The magnetic head 14 includes a induction coil 18, P1 pole 20, and a second pole P2 22 which is separated from P1 pole 20 by write gap 23. The P1 pole 20, second pole P2 22 and write gap 23 can be considered together to be included in the write head 26. Magnetic flux is induced when current is passed through the coil 18 and then passes through the tip of the P2 22 pole, across the gap 23, through the recording medium (not shown) and returns through the P1 pole 20 to complete the magnetic circuit. The magnetic flux thus acts to write data to the magnetic medium.
Magnetic flux flows in lines which are not straight, and thus tends to spread out slightly as it traverses the gap 23 separating the poles P1 20 and P2 22. The amount of “spread” influences the ATI and is influenced by the shape and configuration of the poles P1 20 and P2 22.
FIG. 5 illustrates a top plan view of the pole tip 50 of the P2 pole 22 as seen from the direction of arrow 5 shown in FIG. 4. A portion of the pole tip 50 is included in the ABS 24. The pole tip 50 includes two basic structural portions, namely a straight portion 52 having a basically rectangular shape ending in the ABS 24, and a flared portion 54, which is basically a truncated triangular shape or a trapezoid. The points at which the two portions are connected or where the straight portion 52 flares out into the flared portion 54 are called the flare points 56. It is important for the proper flow of magnetic flux that the flare points 56 are configured within a preferred range 58 relative to the ABS 24.
As is generally known, the process of forming the P2 pole 22 begins with a wafer stack having a layer of photoresist formed on its surface. A photomask with a pattern of opaque and transparent areas is placed between a light source and the layer of photoresist. The areas that are shielded from the light exposure will remain, and the photoresist areas where light hits the photoresist are stripped to create channels which will be filled with plating material to form the P2 pole. The photoresist thus acts as a kind of mold, or plating frame as it is called. The width of the P2 pole tip thus is largely dependent on the width of the channel in the photoresist which acts as its mold. If the channel can be reduced in width, then the width of the final plated P2 pole piece will also be reduced. Recently materials have been utilized which cause the photoresist to expand and thus reduce the channel width, and thus the width of the final P2 pole. These materials are generally referred as photoresist channel shrinking materials and are general applied as a film over the patterned photoresist and then baked to activate the shrinking process.
The photoresist channel shrinking film referred to above is any suitable film that assists in the shrinking of a channel or trench formed within a patterned photoresist. One suitable film is commercially available from Tokyo Ohka Kogyo (TOK) Co., Ltd. in Kanagawa, Japan, and referred to as SAFIER™ (Shrink Assist Film for Enhanced Resolution) coating; product FSC-9220 GM. SAFIER™ is a trademark of TOK Co., Ltd. Another suitable film is commercially available from Clariant Corporation of Muttenz, Switzerland, and referred to as a RELACS® (Resolution Enhancement Lithography Assisted by Chemical Shrinking) coating; product R-500 may be utilized, for example in the present invention. RELACS is a registered trademark of the Clariant Corporation. Both of these films are water-soluble and removable by applying water after use. These two photoresist channel shrinking films serve the same purpose of shrinking the channel of patterned photoresist when baked, but the way in which these films achieve this result is different.
Diffused acid generated during the photoresist exposure remains in the vicinity of the sidewall. Using SAFIER film, during the baking process, this residual acid in the photoresist will diffuse into the channel and act to shrink the SAFIER material, which pulls the photoresist material with it and thus causes the channel to reduce in width. After the SAFIER film is dissolved in the subsequent water rinse, the spacing within the channel determines the width of the patterned element to be finally obtained, and the decrease in the spacing within the channel contributes to reducing the width of the element.
Alternatively, when using RELACS film, after exposure and development of the photoresist to produce the patterned resist, acids are also released from sidewalls within the channel of the patterned photoresist. These acids at the vicinity of the photoresist sidewall diffuse into and react with RELACS. These acids make part of the RELACS material non-soluble in water, and thus not removable from the channel. This also produces a reduction in width of the channel, but by a different mechanism than used by SAFIER.
For the sake of simplifying the present discussion, the photoresist in the channel will be spoken of as “expanding” by whatever means and the channel referred to as “:shrinking”. Thus the term “shrink film” will be used to include materials such as SAFIER and RELACS which shrink the channel, and thus the eventually fabricated pole piece, while portions of the patterned photoresist will expand. It will also be understood that either or both SAFIER and/or RELACS or other materials which act in a similar manner can be used, as will be understood by those skilled in the art.
If the expansion of the photoresist material using these processes were perfectly linear, this might produce the ideal result desired. However, due to the existence of corners and other geometrical features of the photoresist channel, the walls of the expanded photoresist do not move linearly, but instead tend to bulge in places, producing a configuration more closely resembling the distorted expanded photoresist 93 shown in FIG. 16. This produces a distorted channel 94 having distortions 91, such as convex walls 95. If P2 pole plating material were used to fill this distorted channel 94, a distorted P2 pole would be produced. The flare points 56, which ideally would be positioned with the preferred range 58, would thus be either moved out of the preferred range 58, or perhaps not even recognizable or locatable. FIG. 16 shows an example where the walls are so curved that the identification of a “flare point” as a discreet inflection point is not possible.
The flare points discussed are a crucial feature, but not the only crucial feature in the geometry of the write head. Other geometric factors will also be affected by the distortions in the expanded photoresist that may cause problems in the operation of the write head and thus of the disk drive as a whole.
Thus there is a need for a method of fabrication which corrects for distortions in the expanded photoresist and thus in the P2 pole tip produced from the photoresist channel shrinking film process.